Thursday, June 10, 2010

Homemade Wind Sock

Many different materials can be used to make homemade wind socks. The bags new gliders come in is the same material that wings are made out of and can make a couple of nice little socks if you don't mind cutting it up. I've made them out of old tent material, and a cut-up kid's kite. In fact you can use just about any lightweight nylon, or ployester you may find lying around, or just buy a yard or two from the fabric store. Polyester flag bunting was recommended on one site.

Don’t sweat the shape overmuch... this isn't rocket science.

Well, actually, NASA has developed mathematical formulae for aspect ratio and taper, and the document FAA SPECIFICATION FOR WIND CONE ASSEMBLIES,http://www.faa.gov/airports/resources/advisory_circulars/media/150-5345-27D/150_5345_27d.pdf
is six-pages of advisary notes detailing size, fabrication, photometric requirements, specifications on taper for their socks. Here's an excerpt: "The taper or the fabric windsock from the throat to the trailing end must be designed to cause the windsock to fully extend when exposed to a wind of 15 knots (28 km/hr or 17 mph)... and etc... and blah blah."

So it really is rocket science, it's just that, for our purposes, it's EASY rocket science.

I mean come on, little kids make these things out of construction paper.

Just freehand out the shape, cut with scissors, hem the sides and ends, and then turn it inside out and run a line of stitches down the length. Sew seams on the top and bottom. The scoop hole should be about twice the diameter of the exit hole. If you want, you can add a few strips to make streamers to give an extra bit of length and noticability.

Your choice of material will affect your windsock's performance. Ideally, you want a "steady" wind sock when there is a steady breeze. If you don't, then take it home again and change the aspect ratio or the taper or both. (A large inlet hole relative to a short length means a moderate aspect ratio.) Paraglidingforum.com had some tips on windsocks which I list below:

-Very little taper is needed to get a steady wind sock, if and when the fabric is light and non- porous.
-If a wind that should be able to lift the wind sock, can not, then make it shorter.
-If the wind sock flaps, make it narrower towards the outlet hole. Flapping causes an unnecessary increase in wear and tear.
-For larger socks, softer and heavier materials (as opposed to crispy, ripstop nylon), will better withstand higher winds. However, with heavier material, more taper is needed to "inflate" and lift the wind sock.

To hold the scoop end open I used a length of coaxial bicycle brake cable. The cable is usually in three parts: strip off the rubberized coating and discard. The cable itself is made of braided wire inside a flexible metal sleeve. The wire is what you want. But keep a tiny 1/4 - 1/2" piece of the metal sleeve material to connect the two ends into a loop. (Once you thread it into the sewn hem of the scoop end of your finished wind sock.)

Feed alternate ends of your wire into that bit of sleeve material, and seal it up with JB Weld, epoxy, or some other kind of heavy duty adhesive. If the sock I'm making is less than 12 inches in diameter, as is the sock below, (This mini-sock is only 6" in diameter) I might untwist a strand, or two, or three to reduce the effective guage of the braided wire and make it a little bit lighter.

Where do you get bike cable for free? A bike repair shop may have a few feet lying around that you can have for the asking. Every day, I drive by bikes that have been stripped of parts, and have been abandoned by their previous owners. These sad, rusting, skeletonized relics, probably still have some brake cable atatched, and after a point become legitimate salvage as far as I'm concerned.

Here you can see some construction dtetails, where the cable is looped into itself, (cemented into a 1/4" piece of the flexible metal sheath from the bike cable), after being threaded into the hem at the sock's throat end.

You can also see one of three grommets I set to attach string, but this step can be eliminated if you don't have a simple grommet setting tool and grommets.

On the base, next to the pile of streamers sewed onto the narrow end of the sock taper, is an old glasses case where I keep this pocket-sized wind sock, and a mini-telescoping pole (see list) to ground mount this sock 24 inches off the ground, or attach to a car aerial.

If you can't seem to find bike cable, you can also try cutting a thin hoop of flexible plastic from a plastic jug, and thread this through the hem at the throat of your sock to make a semi-rigid opening, but the beauty of this spring steel cable, is that you can fold the scoop end opening into a figure eight, and fold the two ovals back on themselves in the same way a photographer folds up a light reflector.

This halves the diameter of the sock for easy storage, and the cable always pops back into a large, perfect circle again, every time.

A final touch is to add a fishing swivel to the end of the three stings. This swivel clips to the swivel on the end of your pole, or other sock mount, and helps keep the sock cords from tangling.

Another technique, for a larger sock, is to use four, 6" wire fishing leaders. Three of them are attached to the sock itself, and connected to a fourth where the three ends come together. Easy, and the wire leaders never tangle up like string does sometimes.

My main wind sock is one meter long. This mini-wind sock is designed to fit in a pocket. The sock itself is 12" long with four 12" long streamers. The throat opening is 6" in diameter, which folds down to a 3" diameter, that fits with the 24" telescoping mini-pole into a typical case for glasses. The fold is familiar to anyone who has folded a photgrapher's light reflector. Grap the wire as shown below - the 6" hoop into a figure-8, and then fold the two half-sized hoops of the eight onto themselves, to make a 3" diameter hoop which fits readily into a small case.I like adding a few streamers to windsocks, because it helps to identify the tip end at a distance when you are up in the air. It's not uncommon for wind direction to shift even as much as 180 degrees in a short time, and some long windsocks, and one-piece streamers, while visible from the air, may not reveal actual direction till some altitude is given up.